The cellular expression of immune response (Ir) gene function was studied in an in vitro system of primary antibody responses to the TNP conjugates of (T,G)-A--L and (H,G)-A--L. These responses require the participation of T cells and accessory cells as well as B cells, and are under the control of genes mapped to the K or I-A subregions of the H-2 complex. It was demonstrated that the function of accessory cells in responses to TNP-(T,G)-A--L and TNP-(H,G)-A--L is under the control of genes which also map to K or I-A. In contrast, both B cells and T cells from nonresponder H-2a strains to TNP-(T,G)-A--L are competent in supporting primary antibody responses to this antigen, and therefore do not appear to express the Ir gene defect present in these strains. Subsequently, in vitro augmented primary responses to TNP-nuclease (TNP-NASE) have been established and documented to be under the control of H-2 linked Ir gene(s). For these responses as well, accessory cell function was shown to be under Ir gene control. Through the use of intra-H-2 recombinant strains, the Ir gene(s) controlling responsiveness to TNP-NASE were shown to map either to I-B or to complementing genes in subregions including at least I-A and I-E/C.